[lisper]

> no power supply

No internal power supply. Very important distinction.

"The device, a passive cavity resonator, became active only when a radio signal of the correct frequency was sent to the device from an external transmitter."

But the Thing was a transmitter. A receiver could be entirely passive and very small. It could easily fit in a dental implant. (There have been reports of people hearing AM radio broadcasts through their fillings.) But the risk of detection would be very high.